A segmental retaining wall isn't designed to ACI 318 at all — the governing document is the NCMA Design Manual for Segmental Retaining Walls, a factor-of-safety method with its own three distinct stability checks, including an internal sliding check unique to dry-stacked block construction.
Every check above runs automatically inside the Structyze Segmental Block Retaining Wall module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
The Example We'll Use
- Wall height 2 ft; unit 6 in high × 12 in deep, setback 0.75 in
- Backfill γ = 120 pcf, φ = 30°; no surcharge
- Foundation friction φ = 36°; masonry reduction factor 0.7
- Unit density 129 pcf; interface adhesion 0.685 klf, δ = 40°
- Allowable bearing 3.0 ksf; minimum FS 1.5 (all three stability checks)
Working Through It
Three separate stability checks, each about a different failure surface. External overturning about the toe: FS = 153/53 ≈ 2.9, comfortable. Base sliding, using a masonry-friction reduction factor δb = 0.7 applied to the wall's own weight: FS = 131/79.9 ≈ 1.64 — noticeably tighter.
Internal sliding — unique to a dry-stacked block system. Checks the lowest unit-to-unit interface using real ASTM D6638 shear test data (interface adhesion plus friction): Vu = 685 + 258tan(40°) ≈ 902 lb/ft, giving FS = 902/79.9 ≈ 11.3, enormous margin.
Eccentricity and bearing both clear easily: e = 0.11 ft against a middle-third limit of 0.17 ft, and toe bearing of just 0.43 ksf against 3.0 ksf allowable.
What Governs, and Why
Base sliding governs at FS = 1.64, the tightest of the three stability checks even though it still clears the 1.5 minimum — a pattern typical for a conventional gravity SRW, and the reason walls taller than about 4–5 ft generally can't be stabilised by unit mass alone and need a geogrid-reinforced system instead (NCMA Section 7, outside this module's scope).
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Segmental Block Retaining Wall module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
For Segmental Block Retaining Wall and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Why does an SRW need three sliding-related checks instead of one?
Because a segmental wall has two genuinely different sliding failure surfaces — the wall-to-foundation interface (external, base sliding) and the interface between two stacked block courses partway up the wall (internal sliding). Each governed by different friction/adhesion values and requiring its own independent check.
Where do the interface adhesion and friction values come from?
They're product-specific values from full-scale ASTM D6638 shear testing of the actual block unit being used — the defaults shown here are generic placeholders, and a real design must substitute the manufacturer's published test data for the specific block product selected.
Final Thoughts
The internal sliding check is the one really unfamiliar piece of an SRW design if you're used to poured-concrete retaining walls — it doesn't exist in cast construction at all, since there's no unit-to-unit interface to fail along.